The present invention relates to a digital shutter control circuit and, more particularly, to a digital shutter control circuit capable of synchronizing an electric flash device with the actuation of the shutter at an exposure time as short as possible regardless of the series of exposure times set in the shutter.
Heretofore, a digital shutter control circuit has been proposed in which a binary counter circuit activated by clock pulses and having a plurality of output stages each having a different frequency divided time is incorporated and a selector circuit or a mechanical switch is adapted to selectively connect one of the output stages to the shutter operating mechanism thereby permitting the desired exposure time to be obtained corresponding to the selected output stage. In such a digital shutter control circuit, since any of the exposure times is determined by the frequency divided time based on the frequency of the clock pulses, the exposure times are limited to values in terms of 2.sup.n where n is an integer, such as, for example, 1/1000 sec., 1/500 sec.,-1/2 sec. and 1 sec.
On the other hand, in a focal plane shutter having the opening and the closing blade thereof actuated in parallel to the minor sides of frames of 35 mm films, the time required to fully open the shutter by actuating the opening blade is about 7 m sec. and the time at which the closing blade is actuated when the shutter is set at 1/125 sec. is 7.8 m sec. after the opening blade has been actuated as shown in FIG. 1. Therefore, the period during which the shutter is actually held fully opened is about 0.8 m sec. The time period of the mechanical time lag of the closing of the X-contact of the shutter for an electric flash device, after the opening blade has been actuated, is about 0.5 m sec. Therefore, only about 0.3 m sec. is left until the closing blade commences to be actuated after the X-contact is closed. It is thus difficult in such a short time period to fully utilize the illumination of the flash device, i.e., to insure that the intensity of the illumination of the flash device reaches the peak thereof, resulting in uneven exposure of the film if a flash device having a relatively long duration of illumination is used. In order to avoid the above difficulties, the prior art shutter having flash synchronizing X-contact provides a prolonged exposure time of about 1/114 sec. as the nominal exposure time of 1/125 sec. which is within the allowable range in error of time given to the nominal exposure time of 1/125. sec.
However, in a shutter of the type described above utilizing a binary counter circuit, it is impossible to render only the exposure time of 1/125 sec. among the series of exposure times to be slightly longer than the nominal value of 1/125 sec. Therefore, in order to insure proper synchronization of a flash device with the shutter of the type described above, the exposure time must be set to the nomimal exposure time of 1/60 sec. This necessarily makes the range of possibility of using a flash device to be widely limited excluding conditions such as the flash synchronization under the bright daylight condition in which the exposure time must be selected as short as possible.
The present invention is proposed in light of the above described facts.